An Android splash image is just a 320x480 bitmap in R5G6B5 format. But there is a lack of decent tool that will easily create such a file for you. I wrote one up really quickly to fill that gap. This will perform all the resizing, pixel format conversion, and centering for you. You can also preview the image. Pretty handy.

To flash an image:

Start your phone up in Fastboot mode (you must have the Engineering bootloader). You can do this by holding camera and pressing power. Then connect your phone to your PC. Make sure that it is detected as HTC Dream. If you only see "High Composite Device", you will need to install the Android USB Driver. Then press back until it says "Fastboot".

Thanks for de app. I always get a highly compressed image and I don´t know if it´s because of the required format or because your application has some fixed values that could be optimized. Anyway: cool.